In the electrolytic production of fluorine gas (used, for example, in the fluorination of organic substances), commonly used commercial cells comprise an electrolyte-resistant container, a cathode, an electrolyte, a gas separation means, and an anode. The electrolyte-resistant container further comprises a means to maintain electrolyte temperature and a means to replenish hydrogen fluoride consumed during the generation process. The cathode is typically composed of ordinary mild steel, nickel, or MONEL nickel alloy. The electrolyte is typically an approximate composition of KF.multidot.2HF and contains approximately 39 to 42% hydrogen fluoride. See Rudge, The Manufacture and Use of Fluorine and Its Compounds, 18-45, 82-83 Oxford University Press (1962). A gas separation means keeps the generated hydrogen (formed at the cathode) and the generated fluorine (formed at the anode) from spontaneously, and often violently, reforming hydrogen fluoride, see U.S. Pat. No. 4,602,985 (Hough).
The anode used in the electrochemical fluorine cell is typically made of ungraphitized carbon. The carbon can be low-permeability, or high-permeability, monolithic structure, or a composite structure. In a composite structure, there is an inner core of low-permeability carbon and an outer shell of high-permeability carbon formed onto the inner core (see UK Patent Application 2 135 335 A (Marshall)) or otherwise assembled or fabricated (see U.S. Pat. No. 3,655,535 (Ruehlen et al.), U.S. Pat. No. 3,676,324 (Mills), U.S. Pat. No. 3,708,416 (Ruehlen et al.), and U.S. Pat. No. 3,720,597 (Ashe et al.)).
The configuration of the electrode and the characteristics of the materials used therefor determine the efficiency and life of the electrode. Carbon electrodes commonly used as anodes in electrolytic cells are generally a shaped mass of compressed carbon. Typically, commercial anodes have approximately planar or flat surface.
According to Rudge, supra, fluorine generated from a salt melt, such as KF.multidot.2HF, is well known. However, the nature of the electrolytic process is still largely unexplained, although it is known that conditions that exist at or near the surface of the anode are influential on the performance of the anode, see Rudge, supra. When a carbon electrode is immersed into the electrolyte, the carbon is "wetted" by the electrolyte. However, when the electrode is made anodic with reference to another electrode, the carbon is no longer "wetted" by the liquid electrolyte, that is, the "contact angle" increases from about zero to well above 90.degree.. The term "wetted" as used in this application means the spreading of a liquid as a continuous film on a solid, such that the contact angle approaches zero. The term "contact angle" as used in this application means the angle that the surface of a liquid makes with the surface of a solid. Fluorine bubbles at the surface of the anode are lenticularly-shaped and adhere to the surface of the anode.
The forces that lead to poor wetting of the carbon anode by the electrolyte make it difficult for the electrolyte to enter any pores in the anode that may be present until there is sufficient hydrostatic pressure to force it into the pores, see Rudge, supra. For example, carbon that is often used as an anode has a permeability in the range of 0.3 to 3 m.sup.3 air.multidot.m.sup.-2 min (1.0 to 10 ft.sup.3 air.multidot.ft.sup.-2 min) through a 2.54 cm (1 inch) thick plate at 5.0.times.10.sup.2 pascals (Pa) (0.degree. C. and 760 mm Hg of pressure) having internal void volumes of up to 50% or more of the overall volume of the carbon. In the carbon anode, the generated fluorine leaves the anodic surface where it is generated, passes into a reticulated network of pores, passes up through this network, and passes from this network near or above the electrolyte level into the fluorine collection space. It might appear that at significant depths the electrolyte that is forced into the pores by hydrostatic pressure would prevent the fluorine from entering the pores. However, since the electrolyte only poorly wets the carbon, the fluorine gas generated at the surface of the anode has enough energy to displace the electrolyte and enter the reticulated network of pores, as noted above. The electrical resistance of highly porous carbons may be four times that of dense carbon described below. This leads to poorer current density distribution.
According to Rudge, supra, if the carbon anode is fabricated from impervious carbon, that is, low-permeability carbon, the anode also tends to be wetted poorly by the electrolyte. Since there is no appreciable internal reticulated network of pores to escape through, the fluorine gas generated at the surface forms lenticular bubbles on the surface of the anode. As more current is passed through the anode, the bubbles grow and hydrostatic forces force them upward along the anodic surface until they pass into a fluorine collection volume, above the electrolyte surface. As a result, a very large fraction of the anodic surface may be masked by these lenticularly-shaped bubbles. This leads to a reduction of the surface area available to pass electrolytic current into the electrolyte from the anode and generally requires higher voltage operation to obtain the same amount of current. The electrical resistance of low-permeability carbon is only a fraction of that of high-permeability carbon leading to an improved current distribution within the body of the anode.
As discussed in Rudge, supra "polarization" appears to be a problem associated with low-permeability carbon anodes, and to a lesser extent with high-permeability carbon anodes. High-permeability carbon electrodes tend to have a higher threshold to polarization. However, they are intrinsically a poorer conductor than low-permeability carbon, thus high-permeability carbon tends to display a poor current distribution profile. Under constant current operation, the cell voltage will increase, gradually at first and then rapidly until essentially no current will pass through the anode, even at twice the normal voltage. When this happens, the anode is said to be polarized. High voltage treatment is known to provide relief. Various additives and treatments also have been offered to prevent the onset of polarization. For example, see U.S. Pat. No. 4,602,985 (Hough) that describes a carbon cell electrode with improved cell efficiency having smooth, polished surfaces. A method of polishing is also described.
Rudge, supra, further states that in addition to the problems of recovery of the generated fluorine and polarization of the carbon anode, there are several other problems that have been recognized. They include (1) electrical connection between the carbon anode and the current carrying metal contacts, (2) corrosion of the metal at the metal-carbon joint of the electrode, (3) mechanical failure of the carbon anode under uneven mechanical stress; and (4) current distribution up and down the anode.
As noted in Rudge, supra, the first two problems are closely related and should be considered when providing an electrode that will be suspended in an electrolyte. The mechanical and electrical connection between the metal of the current carrying contacts and the carbon anode is subjected to at least two major failure modes. The first failure situation is the mechanical and electrical ability to provide a sound electrical connection. The second failure situation is "bimetallic" or galvanic corrosion at the metal-carbon joint. The area of the carbon anode between the upper surface of the electrolyte and the metal interface of a current collector is subject to resistive heating. This metal-carbon joint corrosion as noted in U.S. Pat. No. 3,773,644 (Tricoli et al.) tends to worsen with the passage of time. During the operation of a cell, high electrical resistance products form at the metal-carbon joint. This is most likely due to vapors developed in the anodic zone above the electrolyte surface and seepage of electrolyte into the metal-carbon joint. These deposits tend to accelerate overheating. Additionally, this leads to accelerated corrosion, accumulations of corrosion products, and the cyclic problem of increased resistive heating due to still higher resistance in the joint.
U.S. Pat. No. 3,773,644 (Tricoli et al.) describes an improved electrolytic cell that is provided with carbon anodes protruding from the cell. The section protruding from the cell is covered by a gas-proof coat made of a good conducting material. The coat is described as consisting of a cap coupled by forcing onto the anode and snugly fitting over and upon the end of the anode.